How does it happen?

Build up of immune complexes along the GBM –> lost of podocyte foot processes –> thickening of the GBM –> sclerosis & hyalinisation of glomeruli

membranous-immunecomplex.pngImage from UNC Kidney Centre

 

What is seen on light microscopy?

Thickening of the GBM with little or no cellular proliferation or infiltration. As the disease progresses – chronic sclerosing glomerular and tubulointerstitial changes develop.

membranous_light

Image from uptodate.com

Long arrows show thickened GBM compared to short arrows showing tubular wall. *=mesangial expansion

 

Silver stain

membranous_silver

Image from uptodate.com

Arrow showing ‘spikes’ along the GBM, which are new basement membrane growing in between the sup epithelial deposits.

 

 

IF ?

Diffuse granular pattern of IgG or C3 staining along the GBM.

membranous_if

Image from uptodate.com. 

Diffuse granular IgG along the capillary walls

EM?

Subepithelial electron dense deposits on the outer aspect of the GBM, effacement of the foot processes of the overlying podocyte, and expansion of the GBM by deposition of new extracellular matrix between the deposits – known as ‘spikes’.

Primary/Idiopathic Membranous

  • electron dense deposits are exclusively sub epithelial and intramembranous.
  • tubular basement membrane staining is rare
  • IgG deposits are mostly IgG4
  • Staining for PLA2R and C1q
  • Staining for IgG1 and IgG3 and leukocyte invasion of the glom tuft is more in keeping with secondary MN

 

Causes of MN

  1. Specific targets
    1. Transmembrane receptor expressed in podocytes
      • Phospholipase A2 Receptor Ab – PLA2R Ab
      • Thrombospondin type-1 domain containing 7A (THSD7A)
    2. Peptides expressed by podocytes
      • neutral endopeptidase (from mom to child)
    3. Abs directed against other antigens
      • IgG4 against PLA2R etc
  2. Genetics
    1. PLA2R on chromosome 2q24
    2. HLA-DQA1 on chromosome 6p21
  3. SLE
    1. Stains for IgA,M and C1q with sub endothelial and sub epithelial deposits and tubuloreticular structures in the glomerular endothelial cells
  4. Drugs
    1. Penicillamine
    2. Gold
    3. Anti TNF agents
    4. NSAIDS
  5. Infections
    1. Hepatitis B with hypocomplementanaemia
    2. Hep C but less so
    3. Syphillis
  6. Malignancy
    • mostly solid tumours – prostate/lung/GI tract
    • less commonly haematological malignancy – CLL
      • deposition of tumor antigens in the glomeruli promotes antibody deposition and complement activation, leading to epithelial cell and GBM injury, and consequent proteinuria
      • in situ binding of antibodies to podocyte antigens – THSD7A & PLA2R
  7. IgG4 disease

 

and lastly MN can also be seen in conjunction with:

  1. Diabetes
  2. Crescentic GN – ANCA/GBM
  3. FSGS
  4. IgA

 

Tests to be carried out if kidney biopsy shows MN

  1. PLA2R Ab tires
  2. Complement (should be normal in just MN, but affected in SLE/Hepatitis)
  3. ANCA/dsDNA
  4. Malignancy screen
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